1. Field of the Invention
The present invention relates to a method for making a hot-rolled steel sheet and an apparatus therefor.
2. Description of the Related Arts
In general, a hot-rolled steel sheet is produced by the following continuous hot rolling process. A slab is heated to a given temperature in a heating furnace. The heated slab is rolled into a rough bar having a specified thickness with a rough rolling mill. Both ends of the rough bar is heated with an edge heater. Thereafter, the rough bar is finish-rolled into a hot-rolled steel sheet having a predetermined thickness in a continuous hot finish rolling mill having a plurality of stands. The hot-rolled steel sheet is cooled on a cooling stand provided on a run-out table and coiled with a coiler.
In the continuous hot rolling process, shape defects, such as "upper curl" and "edge wave", may form at the front end of the rough bar rolled into a given thickness by the rough rolling mill. If the above shape defects form in the rough bar, the rough bar will be damaged due to collision of the rough bar with the edge heater during heating.
Also, the temperature distribution of the rough bar in the transverse direction is nonuniform for the reasons, such as asymmetrical temperature distribution of the slab generated during slab heating, asymmetrical temperature distribution of the rough bar caused in the rough rolling, and temperature decrease at both edge sections of the rough bar.
Nonuniform temperature distribution of the rough bar in the transverse direction causes occurrence in edge wave and center buckle during finish rolling and decrease in finish rolling temperature at the edge sections lower than a phase transformation temperature of the steel, resulting in a decreased yield.
A heating furnace for the slab is provided with a transfer mechanism for transferring slabs fed into the furnace, and the slabs in the furnace are supported by walking beams. The interior of each walking beam is cooled with water to a temperature lower than that of the slab.
In the slab discharged from the heating furnace, the sections being in contact with the walking beams have temperatures lower than those in other sections which does come in contact with the walking beams. The low temperature sections are present at the same frequency as that of the walking beams and called skid marks. When rolling a slab having such skid marks, the shape and quality change at the skid mark positions, resulting in quality deterioration.